1. Field of the Invention
This invention relates to an antenna unit comprising an antenna having a radiating front surface, a back surface, and a lateral outline having at least one electric field reinforcing region.
The invention is applicable in particular to an airborne radar antenna unit adapted for detecting and locating weather conditions.
2. Description of the Related Art
Aboard an aircraft, weather radars are accommodated in the front tip. They generally include a slot antenna, mobile in elevation and azimuth, so as to scan all of the space situated in front of the aircraft.
This antenna is covered by a radome forming the outer surface of the front tip of the aircraft, thereby providing the shape thereof. The radome is electromagnetically transparent so that the antenna can transmit and receive electromagnetic waves. The object of the radome is to protect the radar antenna especially from the direct effects of lightning. For this purpose, the radome includes conductive lightning arresting bands uniformly arranged on the surface thereof, so that in case of lightning on the front tip of the aircraft, the lightning current will evacuate through the bands in order to avoid lightning attachment to the antenna via the radome.
The slot antenna has a general disk shape and has on the lateral outline thereof a stepped profile. However, such steps have salient angles forming electric field reinforcing regions likely to cause peak effects. Indeed, the electric field around the antenna being at a maximum near such salient angles, the latter are likely to become lightning attachment points, and thus responsible for the antenna to be struck by lightning.
Lightning resistance of the front tip of the aircraft is subject both to the dielectric characteristics of the radome component material, the ensured minimum spacing between the antenna and the inner wall of the radome, independently of the position of the antenna, and the setup of lightning arresting bands distributed over the periphery of the radome.
In particular, ensuring minimum spacing between the antenna, which is metallic, and the inner wall of the radome allows for the electric field developing near the antenna under lightning conditions to be limited.
Rating of an aircraft in relation to direct lightning resistance is obtained by means of standardized tests. Once acquired, this rating is transferred from one aircraft to the next as long as the configuration of the aircraft with regard to lightning resistance remains unchanged, i.e. as long as the characteristics conditioning lightning resistance of the aircraft remain unchanged. However, as soon as any of such characteristics is modified, the lightning resistance of the front tip of the aircraft is modified. A new rating is then required which implies that all or some of the rating tests must be repeated.
In particular, the characteristics determining lightning resistance are dependent on the characteristics of the weather radar employed, especially the dimensions of the antenna, positioning thereof inside the radome, and scanning in space which has to be done by the antenna. However, such characteristics of the radar are not standardized and may be modified in order to improve the performance thereof. Namely, the position of the antenna can be advanced inside the radome, and/or the diameter thereof can be increased, thus increasing scanning amplitude.
Such modifications will lead to deterioration in lightning resistance of the front tip of the aircraft.
In order to maintain the same spacing between the antenna and the radome, so as to keep the rating achieved for the aircraft with regard to lightning resistance, it may be envisaged to shift the entire radar backward or increase the radome. However, such solutions are not always compatible with the geometry of the aircraft nose and the interior design thereof.
The increase of the radome's electric strength, which allows for dielectric resistance, and thus electric field resistance, to be improved, can also be envisaged. However, this approach implies an increase in the thickness of the radome, i.e. the mass thereof, and a decrease in radio performance resulting in a negative impact on the performance of the radar, and more generally speaking of the aircraft.